As one of the known arts of an occupant load sensor, a strain sensor is disclosed in Japanese Patent No. 2003-83707A (hereinafter, referred to as reference 1). According to the strain sensor in reference 1, a detecting member is press-fitted into a detecting hole provided at an intermediate portion of the sensor substrate (strain member), and first and second fixing members are press-fitted into first and second fixing holes provided at both sides of the detecting hole, respectively. In addition, first and fourth strain detecting elements are provided between the detecting hole and the first fixing hole, while second and third strain detecting elements are provided between the detecting hole and the second fixing hole. Then, first to fourth strain detecting elements are connected by circuit patterns, so that a Wheatstone bridge is structured. An output from the Wheatstone bridge circuit is connected to an amplifying circuit provided at an upper surface of the sensor substrate (strain member). An output from the amplifying circuit is connected to an output electric pole provided at an end portion of the sensor substrate with an external power electric pole and an external GND electric pole.
According to the strain sensor disclosed in the reference 1, when an external force is applied to the detecting members, a component force is applied to the first and second fixing members and a strain is generated to the sensor substrate. The strain is detected as a resistance change of each strain detecting element. The resistance change is transformed to electric voltage in the Wheatstone bridge. In addition, an output electric voltage from the Wheatstone bridge is amplified by an amplifying circuit and outputted to an outside as a sensor output via the external output electric pole.
However, according to the strain sensor disclosed in the reference 1, not only the Wheatstone bridge circuit but also wiring patterns, for connecting the Wheatstone bridge with the external power electric pole and the external GND electric pole with the external output electric pole, are formed on the sensor substrate. Therefore, when the sensor substrate is deformed receiving the strain, the wiring patterns formed on the sensor substrate are also deformed. Accordingly, there is a possibility that the sensor output from the external output electric pole is influenced. Further, the strain detecting elements on the sensor substrate and the wiring patterns are desired to be arranged so as not to be in contact with one another, which may cause a need for a large space.
A need thus exists for an occupant load detecting sensor of a seat for a vehicle which is not susceptible to the drawback mentioned above.